People who know Sun An Tso, the 18-year-old Taiwanese exchange student charged in the US with threatening to shoot up his school, say he liked guns and flame throwers and had dreams of a police career.

Sun who has been jailed in the state of Pennsylvania, was “a child who really cares about things,” said Cheng Wei-ting, a tutor who home-schooled Sun in Taiwan’s main city, Taipei.

“He was an extremely simple and kind student, yet he would often have unusual ideas,” Cheng said.

Sun idealised the famed Taiwanese-American forensics scientist Lee Chang-yu and wanted eventually to study criminal psychology at an American university, his father’s friend Tuo Zong-kang told reporters on Thursday.

“He planned to study in the same fields as Lee, such as criminal psychology and forensics,” Tuo said.

Sun threatened to commit a shooting on May 1 at the Monsignor Bonner and Archbishop Prendergast High School he attended, said police in the Pennsylvania town of Upper Darby.

They said he had live ammunition, a military-style ballistic vest, ammunition clip pouches and a high-powered crossbow in the home where he was staying in the town of Lansdowne, and had researched how to buy weapons.

Sun later said his threat was only a joke, but he was arrested on Tuesday and has been charged with making terroristic threats.

Defence lawyer Enrique Latoison said Wednesday that his client “had no intention or plans” to commit a school shooting and that many items found were what he wore to school for a Halloween costume contest. He said his client said he was “just kidding.”

Sun’s latest Facebook post, from October, shows a person dressed in army fatigues, headphones and a brown ski mask carrying a scoped rifle.

A February 2017 post that was also his Facebook background cover photo shows orange flames being fired beside a cement wall and stairwells leading into a ravine.

“What I recall most is that when these school shootings happened in America, he would tell me that if he had been on the scene he would definitely be the first to protect his classmates, so he needs to have a strong physique,” said Cheng, who added that she once helped him research how to join a police Swat team.

Sun’s parents, actor and assistant film director Sun Peng and actress Di Ying, could not be reached for comment. But Di told Taiwanese broadcaster Sanlih E-Televisionthat her son was a “military affairs fan” who was “making a joke” about his school in the US.

Tuo, his father’s friend, said he remembered Sun as “a very polite kid who had good grades, very disciplined and very kindhearted.”

“He lives in a family full of warmth, and his parents are very kind to him,” Tuo said. Regarding his arrest, he said, “I think it is a very, very big misunderstanding.”

Before moving to Pennsylvania, Sun completed two years of high school at the private Kang Chiao International School in New Taipei City, publicity head Wu Hao-ming said.

Before that, he achieved average grades at the private Wego Elementary School in Taipei, where he was popular among peers and teachers and showed no interest in guns, academic director Liu Jen-Hao said.

“So this matter [in the United States] we find rather surprising,” Liu said.

Sun was taken to Delaware County Prison in lieu of US$100,000 bail. A preliminary hearing is scheduled for April 11.

Sun’s case comes as school shootings in the United States, including the deaths last month of 17 students and staff members in Parkland, Florida, have triggered some of the biggest youth protests in recent years and calls for stronger gun control measures.